Multicomponent plasmonic photocatalysts consisting of a plasmonic antenna and a reactive catalytic surface: the antenna-reactor effect

ABSTRACT

A method of making a multicomponent photocatalyst, includes inducing precipitation from a pre-cursor solution comprising a pre-cursor of a plasmonic material and a pre-cursor of a reactive component to form co-precipitated particles; collecting the co-precipitated particles; and annealing the co-precipitated particles to form the multicomponent photocatalyst comprising a reactive component optically, thermally, or electronically coupled to a plasmonic material.

STATEMENT REGARDING FEDERALLY SPONSORED RESEARCH OR DEVELOPMENT

The invention was made with government support under Grant No.FA9550-15-1-0022, awarded by the Air Force Office of Scientific Researchand Grant No. DGE1450681, awarded by the National Science Foundation.The government has certain rights in the invention.

BACKGROUND

Industrial processes depend extensively on heterogeneous catalysts forchemical production and mitigation of environmental pollutants. Theseprocesses often rely on metal nanoparticles dispersed onto high surfacearea support materials to both maximize catalytically active surfacearea and for the most cost-effective use of expensive catalysts such aspalladium, platinum, ruthenium, or rhodium. However, catalytic processesutilizing transition metal nanoparticles are often energy intensive,relying on high temperatures and pressures to maximize catalyticactivity.

Light-driven chemical transformations may offer an attractive andultimately sustainable alternative to traditional high-temperaturecatalytic reactions. Metallic plasmonic nanostructures may be useful forphotoactive heterogeneous catalysts. Plasmonic nanoparticles uniquelycouple electron density with electromagnetic radiation, leading to acollective oscillation of the conduction electrons in resonance with thefrequency of incident light, known as a localized surface plasmonresonance (LSPR). These resonances lead to enhanced light absorption inan area much larger than the physical cross-section of the nanoparticle,and such optical antenna effects result in strongly enhancedelectromagnetic fields near the nanoparticle surface. An LSPR can bedamped through radiative reemission of a photon, or non-radiative Landaudamping with the creation of energetic “hot” carriers: electrons abovethe Fermi energy of the metal and/or holes below the Fermi energy.

In this context, “hot” refers to carriers of an energy that is asignificant fraction of the plasmon energy that would not be generatedthermally at ambient temperature. Plasmonic metal nanoparticles havebeen shown to induce chemical transformations directly on theirsurfaces, through either phonon-driven or charge-carrier-drivenmechanisms in Au, Ag, Cu, and, recently, Al nanoparticles. Althoughthese “good” plasmonic metals may show initial promise forplasmon-induced photocatalytic chemistry, in general they have beenshown to not be universally good catalytic materials.

In comparison, non-coinage transition metals have historical precedenceas excellent catalysts, yet are generally considered poor plasmonicmetals, because they suffer from large non-radiative damping, whichresults in broad spectral features and weak absorption across thevisible region of the spectrum. Many catalytic transition metalnanoparticles (Pt, Pd, Rh, Ru, etc.) possess LSPRs in the UV, but thisis disadvantageous for photocatalysis because of poor overlap withconventional laser sources or, alternatively, with the solar spectrum.One option to increase transition metal nanoparticles absorptionproperties is to increase the transition metal nanoparticle size, whichwould redshift the optical absorption, but it also increases cost andreduces surface area, and therefore catalytic activity.

This invention was made with support from the following Welch FoundationGrants: C-1220 and C-1222.

SUMMARY

This summary is provided to introduce a selection of concepts that arefurther described below in the detailed description. This summary is notintended to identify key or essential features of the claimed subjectmatter, nor is it intended to be used as an aid in limiting the scope ofthe claimed subject matter.

In one aspect, embodiments disclosed herein relate to a method of makinga multicomponent photocatalyst, that includes inducing precipitationfrom a pre-cursor solution comprising a pre-cursor of a plasmonicmaterial and a pre-cursor of a reactive component to formco-precipitated particles; collecting the co-precipitated particles; andannealing the co-precipitated particles to form the multicomponentphotocatalyst comprising a reactive component optically, electronically,or thermally coupled to a plasmonic material.

In another aspect, embodiments disclosed herein relate to a method ofcatalyzing a reaction that includes forming a multicomponentphotocatalyst pre-cursor, by a method that includes inducingprecipitation from a pre-cursor solution comprising a pre-cursor of asupport material, a pre-cursor of a plasmonic material, and a pre-cursorof a reactive component to form co-precipitated particles of themulticomponent photocatalyst pre-cursor; and collecting themulticomponent photocatalyst pre-cursor; loading the multicomponentphotocatalyst pre-cursor into a high-temperature reaction chamber;annealing the loaded multicomponent photocatalyst to form amulticomponent photocatalyst comprising a reactive component optically,electronically, or thermally coupled to a plasmonic material; feedingreactants into the reaction chamber; and illuminating the multicomponentphotocatalyst in the reaction chamber with a light having a wavelengthoverlapping a plasmon resonance of the plasmonic material.

Other aspects and advantages of the claimed subject matter will beapparent from the following description and the appended claims.

BRIEF DESCRIPTION OF DRAWINGS

FIG. 1 shows graphical models and calculated near-field enhancement forboth Pd—Al NC and Pd—Al₂O₃NC systems.

FIG. 2 shows a high resolution transmission electron microscope (HRTEM)image showing a chemically synthesized Pd—Al NC.

FIG. 3 shows the optical properties of the chemically synthesized Pd—AlNC with increasing Pd coverage as predicted by theory (right) and asobtained experimentally (left).

FIG. 4 shows photocatalytic activity for the hydrogen-deuterium exchangereaction using Pd—Al NC and Pristine Al NC.

FIG. 5 shows the quantitative consumption of H₂ at the dipolar LSPR ofPd—Al NC.

FIG. 6 shows the excitation laser power dependence of thehydrogen-deuterium exchange reaction using the Pd—Al NC multicomponentphotocatalysts measured at 492 nm and 800 nm, corresponding to thedipolar plasmon resonance and Al interband transition, respectively.

FIG. 7 shows temperature dependent reaction activity measurements forthe hydrogen-deuterium exchange reaction at temperatures between 300 Kand 400 K using the Pd—Al NC without external illumination.

FIG. 8 shows a plot of the selectivity of acetylene reduction using thePd—Al NC as a function of temperature or laser power density.

FIG. 9 shows representative gas chromatogram yields for ethylene forboth photo-hydrogenations of acetylene (A) and thermal hydrogenations ofacetylene (B) using the Pd—Al NC multicomponent photocatalysts.

FIG. 10 shows TEM images of Al@Cu₂O particles formed by chemicalsynthesis.

FIG. 11 shows plots showing results for the optical characterization ofAl NCs, Al@Cu₂O, and Cu₂O.

FIG. 12 shows (a) gas chromatogram results of photocatalytic CO₂hydrogenation experiments, via the reverse water-gas shift reaction(rWGS) and (b) the spectrum of the light source used for illumination.

FIG. 13 shows plots of the photocatalytic activity versus thermal-drivenactivity for the rWGS reaction using the Al@Cu₂O multicomponentphotocatalysts and pristine Al.

FIG. 14 shows (a) the rate of CO formation as a function ofvisible-light intensity under ambient conditions and (b) the externalquantum efficiency (EQE) as a function of photon flux when using theAl@Cu₂O multicomponent photocatalyst or pristine Al.

FIG. 15 shows a plot of the measured EQE for Al@Cu₂O and pristine Alversus illumination wavelength.

FIG. 16 shows a plot of the H₂ production rate during photocatalysis(9.6 W/cm² white light illumination) and thermocatalysis (at 482° C.)for the various catalysts.

FIG. 17 show a plot demonstrating the reaction rate during multiple hourlong measurement of photocatalytic rates using the Cu—Ru surfacealloy@Cu supported on MgO—Al₂O₃ catalyst under 9.6 W/cm² white lightillumination without external heating.

FIG. 18 shows a plot comparing the photocatalytic and thermocatalyticrates using the Cu—Ru surface alloy@Cu supported on MgO—Al₂O₃ catalyst.

FIG. 19 shows a high-resolution transmission electron micrograph (TEM)of a single Cu—Ru surface alloy particle according to an embodiment ofthe application.

FIG. 20 shows a high-angle annular dark-field (HAADF) image of thereduced Cu—Ru surface alloy (lighter contrast areas) supported on theMgO—Al₂O₃ support according to an embodiment of the application.

FIG. 21 shows a plot of the size distribution of the reduced Cu—Rusurface alloy of Example 3.

FIG. 22 shows a HAADF image of the reduced Cu nanoparticles (lightercontrast areas) supported on the MgO—Al₂O₃ support according to anembodiment of the application.

FIG. 23 shows a plot of the size distribution of the reduced Cunanoparticles of Example 3.

FIG. 24 shows a HAADF image of the reduced Ru nanoparticles (lightercontrast areas) supported on the MgO—Al₂O₃ support according to anembodiment of the application.

FIG. 25 shows a plot of the size distribution of the reduced Runanoparticles of Example 3.

FIG. 26 shows UV-Vis diffuse reflectance spectra of Cu—Ru surface alloy(solid line), Cu nanoparticles (dashed line) and Ru nanoparticles(short-dashed line).

FIG. 27 shows powder X-ray diffraction (PXRD) of Cu—Ru surface alloy onMgO—Al2O3 support and XRD data of Cu, Ru, MgO and Al2O3 fromInternational Centre for Diffraction Data (ICDD) cards.

FIG. 28 shows X-ray photoelectron spectroscopy (XPS) result of Cu—Rusurface alloy supported on MgO—Al₂O₃ and Ru nanoparticles supported onMgO—Al₂O₃.

FIG. 29 shows a plot of highest surface temperatures and average surfacetemperatures of a sample pellet of the Cu—Ru surface alloy supported onMgO—Al₂O₃ under white light illumination as a function of lightintensity.

FIG. 30 shows two Arrhenius plots of apparent activation barriers fordifferent wavelengths (upper plot) under constant intensity of 3.2 W/cm²and in the dark (trend line not marked by a wavelength) and (lower plot)for different light intensities at 550 nm and in the dark (trend linenot marked by a wavelength).

FIG. 31 shows the wavelength dependence of photocatalytic reaction rateon Cu—Ru surface alloy@Cu supported on MgO—Al₂O₃.

FIG. 32 shows the apparent activation barriers (eV) under variousillumination conditions for the Cu—Ru surface alloy@Cu supported onMgO—Al₂O₃.

FIG. 33 is a table showing the element concentration in thecoprecipitated precursor (i.e., before the activation process) asmeasured by inductively coupled plasma (ICP) spectroscopy.

FIG. 34 shows the reaction rate and long-term stability duringphotocatalytic dry methane reforming reaction under 19 W/cm² white lightillumination.

FIG. 35 shows the selectivity and long-term stability duringphotocatalytic dry methane reforming reaction under 19 W/cm² white lightillumination.

FIG. 36 shows the long-term stability (solid circles) and selectivity(open circles) of photocatalysis under 19 W/cm² white light illuminationand thermocatalysis at 1000 K reactor temperature when using theCu_(19.8)Ru_(0.2) catalyst.

FIG. 37 shows a plot of the intensity dependence of the photocatalyticreaction rate and selectivity using the Cu_(19.8)Ru_(0.2) catalyst.

FIG. 38 shows a plot of the wavelength dependence of the photocatalyticreaction rate and selectivity using the Cu_(19.8)Ru_(0.2) catalyst.

DETAILED DESCRIPTION

In general embodiments disclosed herein relate to multicomponentplasmonic photocatalysts. More specifically, embodiments disclosedherein relate to photocatalysts that include a plasmonic material thatmay act as an optical antenna that modifies and improves the catalyticactivity of a separate and distinct component acting as a reactivecomponent. The multicomponent plasmonic photocatalysts disclosed hereincan be engineered to include a plasmonic material that effectivelyabsorbs light having wavelengths across to solar spectrum to improve thecatalytic activity of a separate reactive component.

As will be discussed further, due to the vast array of possiblecombinations of plasmonic materials with reactive components, theembodiments disclosed herein introduce an unprecedented modularity tothe design and optimization of photocatalytic materials, wherein theparticular selection, and subsequent combination, of a plasmoniccomponent and a reactive component may result in a unique photocatalystthat can operate at milder conditions while also possessing a reactivityprofile with improved efficiency and selectivity.

Without being bound by theory, it is believed that when in operation theplasmonic component of the multicomponent photocatalyst acts as anoptical antenna, capable of absorbing light from a physical area muchlarger than its geometric cross-section due to the unique interaction oflight with plasmonic materials. The unique interaction of light withplasmonic materials is capable of generating strong electric fields onand near the plasmonic material surface as a result of the collectiveoscillation of electrons within the plasmonic material. This oscillationis known as a plasmon, and in the presently described multicomponentphotocatalyst concept the strong electric field from the plasmonicmaterial is optically coupled to the reactive component inducing apolarization, or “forced plasmon”, within the reactive component. Theoptical coupling of the plasmonic material with the reactive component,that is the generation of a forced plasmon in the reactive component asa result of a plasmon on the plasmonic material, may occur even when theplasmonic material and the reactive component are separated by distancesof up to about 30 nm.

The forced plasmon induced in the reactive component rapidly decays intoenergetic hot-carriers in the reactive component and these hot-carriersenable reactions to occur between adsorbate molecules on the reactivecomponent surface under milder conditions than traditionally used duringcatalysis. In general, an optimal reactive component is not as effectiveat absorbing light as the plasmonic component and, thus, the combinationof both the plasmonic component and the reactive component can synergizeeach of the components most useful functions (e.g., absorption orreactivity) into a modular multicomponent photocatalyst capable ofoperating as a photocatalyst more efficiently than each component on itsown.

In one or more embodiments, the reactive component may also beelectronically coupled to the plasmonic material. Specifically, hotcarriers can be generated in the plasmonic material through plasmondecay and transfer to the reactive component to further drive chemicalreactions for catalysts that are electronically conductive between theplasmonic material and the reactive components. In one or moreembodiments, the reactive component may also be thermally coupled to theplasmonic material. Specifically, plasmonic materials strongly absorblight and converts some of the light energy into heat, which canthermally drive reactions on the reactive components closely associatedtherewith. The increase in local temperature is an advantage ofplasmonic materials compared to other light absorbing materials.

In one or more embodiments, the plasmonic material may be any materialwith free carriers. In particular, the plasmonic material has freecarriers that may include free holes, free electrons, or electrons inthe conduction band. For example, the plasmonic material may be a metal,semiconductor, or a molecule. In one or more embodiments, the plasmonicmaterial may be an insulator or a single-atom species. In one or moreembodiments, the plasmonic material may be in general any metal ormetalloid element on the Periodic Table of the Elements and alloysincluding said elements. In more specific embodiments, the plasmonicmaterial may be, but is not limited to, gold (Au), silver (Ag), copper(Cu), aluminum (Al), and alloys including said elements. In the presentdisclosure the term “alloys” is intended to cover any possiblecombination of metals. For example, the alloys may be binary alloys suchas AuAg, AuPd, AgPd, AuCu, AgCu, etc., or they may be ternary alloys, oreven quaternary alloys. In one or more embodiments, the alloy may be ahomogenous or heterogeneous alloy.

In one or more embodiments, the plasmonic material may be selected fromBi₂Te₃, Mg, ZrN, Bi, graphene, MoS₂, WO₃, ZnO, Pd, Ru, Rh, Pt, In, Ga,Co, Fe, GaN, Cu_(2-x)S, Cu_(2-x)Te, Cu_(2-x)Se, Li, K, Rb, Cs, TiN, ordoped semiconductors including indium tin oxide (ITO), fluorine dopedtin oxide (FTO), or doped silicon. In one or more embodiments, theplasmonic material may be a 2-dimensional material, such as singlemonolayer materials, nanosheets, nanoplates, or thin films. In general,2-dimensional materials may be defined as materials that have twodimensions (e.g., length, width, and height) that are each independentlyat least 10 times the size of the other dimension, or at least 25 timesthe size of the other dimension, or at least 50 times the size of theother dimension, or at least 100 times the size of the other dimension.

In one or more embodiments, the plasmonic material may have at least aportion of its surface coated with a spacer material. A spacer materialmay physically separate or space the plasmonic component from thereactive component. In one or more embodiments, the spacer material maybe a carbonaceous material, a nitride, a phosphide, a silicide, anarsenide, a selenide, a telluride, a hydride, a sulfide, a carbide,metal organic frameworks, covalent organic frameworks, a polymericmaterial, or an oxide. In one or more embodiments, the spacer materialmay be a crystalline material, an amorphous material, or a material thatis a mixture of crystalline and amorphous.

In one or more embodiments, the plasmonic material may have an oxideshell as a spacer material, which surrounds the plasmonic material coreof one of the metals or alloys listed above. In one or more embodiments,the oxide shell may be a natural/native oxide shell that forms upon ametal or alloys after exposure to air or water. For example, a copperplasmonic material may possess a copper oxide (e.g., CuO or Cu₂O) shellsurrounding a copper core, or an aluminum plasmonic material may possessan aluminum oxide shell surrounding an aluminum core. In someembodiments, the oxide shell may be at least partially artificiallyproduced, such as by artificially increasing the thickness of anative/natural oxide shell by appropriate chemical methods, or bychemically synthesizing, or otherwise depositing, an oxide materialaround a pre-formed plasmonic material. In one or more embodiments, thespacer material may have a thickness of up to about 30 nm, or up toabout 25 nm, or up to about 15 nm. In one or more embodiments, thespacer material may have a single atom thickness or a thickness of atleast about 0.5 nm, or at least 1 nm, or at least 1.5 nm. In moreparticular embodiments, the spacer material may have a thickness betweenabout 1 nm and 5 nm.

In one or more embodiments, the plasmonic material may have a plasmonresonance, or optical absorption maximum, in the ultraviolet to infraredregion of the electromagnetic spectrum. For example, in one or moreembodiments, the plasmonic material has a plasmon resonance betweenwavelengths of about 180 nm to 10 microns. In one or more embodiments,the plasmon resonance is at least any value between about 180 nm and 380nm. In one or more embodiments, the plasmon resonance may be at most anyvalue between 760 nm and 10 microns. More specifically, the plasmonicmaterial may have a plasmon resonance, or optical absorption maximum, inthe visible region (e.g., at a wavelength between about 380 nm-760 nm)of the electromagnetic spectrum. Those with skill in the art willappreciate that, in addition to the material's elemental composition,the size and shape of the plasmonic material, as well as theenvironment/medium that the plasmonic material is in may affect itsLSPR. Therefore, any material having a size and/or shape that canachieve a plasmon resonance, or optical absorption maximum, in theultraviolet to infrared region of the electromagnetic spectrum when inan environment that is substantially air or water is intended to becovered by the present application.

As stated above, a material's elemental composition, its size, and itsshape may all affect its LSPR. As the plasmonic materials describedherein may take different shapes including, but not limited to sheets(e.g., 2-dimensional), wires (e.g., 1-dimensional), rods, cuboidal,spherical, or spheroidal (i.e. approximately spherical), etc. The sizeof the plasmonic material may be a dimension that equates to the longestedge length or to the diameter of a circumscribing sphere for sphericaland spheroidal plasmonic particles. In one or more embodiments, ingeneral the plasmonic material may have at least one dimension with asize between about 1 nm and 300 nm or between about 5 nm and 200 nm.More specifically, for specific metals the plasmonic material may haveat least one dimension with a size as follows: Ag—5 nm-150 nm forvisible LSPR, Au—5 nm-200 nm for visible and IR LSPR, Cu—1 nm-200 nm fora visible LSPR, and Al—10 nm-50 nm for UV LSPR and 50 nm-200 nm forvisible LSPR.

In general, the reactive component may be any compound capable ofcatalyzing a reaction. In one or more embodiments, the reactivecomponent may be a metal, semiconductor, insulator, single atom species,ionic species, organic molecules, metal complexes, or atomic clusterspecies with between 2 and 3×10⁷ atoms. In one or more embodiments, thereactive component may be selected from transition metals, lanthanides,actinides, oxides, sulfides, hydrides, nitrides, carbides, silicides,phosphides, arsenides, selenides, tellurides, anchored ligandscontaining organic and inorganic functionality, metal organicframeworks, or covalent organic frameworks. In one or more embodiments,the reactive component may be any metal or metalloid element on thePeriodic Table of the Elements and alloys, oxides, phosphides, andnitrides including said elements. Further, the reactive component may beany oxides. In one or more embodiments, the reactive component is atransition metal or a transition metal oxide. In one or moreembodiments, the reactive component is a transition metal alloyed at thesurface of the plasmonic material to form a surface alloy particle wherethe bulk of the particle is plasmonic material and substantially all ofthe reactive component is present at the surface of the particle.

More specifically, in some embodiments the reactive component may beselected from nanoparticles of metals including at least one ofpalladium (Pd), platinum (Pt), ruthenium (Ru), rhodium (Rh), nickel(Ni), iron (Fe), cobalt (Co), iridium (Ir), osmium (Os), titanium (Ti),vanadium (V), indium (In) their alloys, their oxides, their phosphides,and their nitrides. Further, in one or more embodiments, the reactivecomponent may be intermetallic nanoparticles, core-shell nanoparticles,and semiconductor nanoparticles (e.g., Cu₂O) including the metal andmetalloid elements of the Periodic Table of the Elements. In one or moreembodiments, the reactive component may be monometallic, bimetallic, ormultimetallic nanoparticle islands, shells, or discrete atomic siteslocated on the plasmonic component. Those with skill in the art willappreciate that, in addition to the reactive component's elementalcomposition, the size and shape of the reactive component may affect itssubstrate adsorption properties, chemical reactivity, and reactionselectivity.

In one or more embodiments, the reactive component may have at least onedimension with a size of at least an atomic diameter of a metal or ion.For example, the reactive component may have at least one dimension witha size of at least 30-300 picometers. In one or more embodiments, thereactive component may have at least one dimension with a size of atmost 100 nm, or at most 75 nm, or at most 50 nm, or at most 25 nm, or atmost 15 nm.

In one or more embodiments, the reactive component may be physically orchemically attached to the surface of the plasmonic component, while inother embodiments the reactive component may be separated by a distancefrom the plasmonic component. The separation may be either by emptyspace (i.e., a distinct physical separation) or the separation may be bya spacer material discussed above. For example, the plasmonic componentand the reactive component of the multicomponent photocatalysts may beseparated by a small distance when they are prepared via lithographicmethods to have a distinct physical separation. In one or moreembodiments, the small separation may be a distance of up to about 30nm, or up to about 25 nm, or up to about 15 nm. In one or moreembodiments, the small separation may be at least about 0.1 nm, or atleast 2 nm, or at least 5 nm or at least 10 nm. In one or moreembodiments, a plurality of reactive components may be physicallyattached to the surface of a single plasmonic component, which canincrease the surface area available for reactions. In one or moreembodiments, the reactive component may form a shell that surrounds,either completely or substantially (e.g., greater than 50%), the surfaceof the plasmonic component.

In general, the reactive component can be used to perform any reactionthat it is capable of performing without it being optically coupled to aplasmonic component. In one or more embodiments, the reactive componentmay be capable of oxidation and reduction chemistry, water or airpollution remediation reactions, NOx and N₂O decompositions, catalyzinghydrogenation reactions, such as acetylene hydrogenation, carbon dioxideconversion to carbon monoxide via the reverse water-gas shift reaction(which can be coupled with a hydrogenation to create hydrocarbons usingFisher-Tropsch synthesis), and nitrogen activation chemistry, includingthe synthesis of ammonia. Additional specific reactions that may beperformed efficiently by the reactive component in multicomponentphotocatalysts described herein may include methane steam/dry reforming,ammonia decomposition, nitrous oxide decomposition, reverse water gasshift, water gas shift, and the selective reduction of acetylene. Whilespecific reactions are indicated above it is to be understood that anycatalytic reaction currently performed using a single reactive componentmay be enhanced by incorporating a plasmonic component and forming amulticomponent photocatalyst as described herein.

In one or more embodiments, the multicomponent photocatalysts may be aplasmonic material that is alloyed at its surface with a reactivecomponent. That is to say that the bulk or core of the plasmonicmaterial is unalloyed and includes only the plasmonic material, while atthe surface (i.e., at least the first layer and up to the first threelayers of the plasmonic material) a reactive component is alloyed withthe plasmonic material. Multicomponent photocatalysts of this type maybe referred to as surface alloys or heterogeneous alloys. In surfacealloys the electronic structure of the plasmonic material and thesurface alloyed reactive component are substantially similar to theelectronic structures expected for each component separately (i.e., in anon-alloyed state) and as a result the plasmonic material maintains astrong LSPR and the reactive component maintains high interaction withsubstrate molecules and high catalytic activity. Moreover, a surfacealloyed multicomponent photocatalyst may significantly improve atomicutilization and thereby reduce cost by segregating the often costlyreactive component specifically at the surface where it is needed forcatalysis. Thus, the amount of reactive component necessary may besignificantly reduced when compared with conventional catalysts thatinclude substantial amounts of reactive component in interior sites thatare not actually available for catalysis.

Surface alloys may be formulated that combine a plasmonically activematerial with a catalytically active reactive component that isatomically dispersed in the surface layer of the plasmonically activematerial. Atomic dispersion is understood to mean that the reactivecomponent is randomly and atomically distributed at surface sites of thesurface alloy particle. Thus, a surface alloy combines two or morefunctional components (e.g., plasmonic and reactive) synergistically ina single structure or discrete particle. In one or more embodiments, asurface alloy multicomponent photocatalyst may include a plasmonicmaterial selected from Al, Ag, Au, and Cu and a reactive componentselected from a transition metal, wherein the reactive component isalloyed with the plasmonic material at the surface. In one or moreembodiments, the transition metal may be selected from Pd, Ru, Rh, Pt,and Ni. In one or more embodiments, the molar ratio of plasmonicmaterial to reactive component in a surface alloy may be between 1000:1to 10:1 or between 400:1 to 20:1. When the amount of reactive componentis too small the reactivity of the multicomponent photocatalyst may betoo low. However, when the amount of reactive component is too higheither a shell or multiple layers of reactive component may form on theplasmonic component instead of a surface alloy. In a more specificembodiment, the plasmonic material may be Cu and the reactive componentmay be selected from a transition metal.

In general, the method of making the multicomponent photocatalysts isnot intended to be particularly limited. In one or more embodiments, themulticomponent photocatalysts may be created using any method thatresults in a plasmonic material having at least one reactive componentphysically or chemically attached thereto or that results in a reactivecomponent that is separated a distance from the plasmonic material. Forexample, the multicomponent photocatalyst may be created via a colloidalmethod wherein the plasmonic material is created first by thedecomposition or reduction of a plasmonic pre-cursor compound (e.g.,aluminum hydride or an organoaluminum compound for aluminum plasmonicmaterials). A transition metal salt, transition metal carbonyl complex,or other reactive component precursor may then be added to a solutioncontaining the plasmonic material (or a pre-cursor compound) andsubsequently or concurrently reduced to form metallic, metal oxide, orsemiconducting reactive component islands/particles or shells on oraround the plasmonic material. The multicomponent photocatalysts thusformed may be isolated by centrifugation or any other method capable ofseparating the multicomponent photocatalysts from solution.

In one or more embodiments, the surface alloys described above may beformed by a co-precipitation process, whereby pre-cursors of theplasmonic material and pre-cursors of the reactive component aredissolved in a liquid to form a pre-cursor solution before precipitationis induced to form intimately mixed co-precipitated particles. In one ormore embodiments, precipitation may be induced by adding the pre-cursorcontaining solution to a basic solution or vice-versa. For example, thepre-cursor solution and the basic solution may be added simultaneouslyor sequentially together in order to induce precipitation. In one ormore embodiments, the pre-cursor solution and the basic solution may beadded together dropwise. In one or more embodiments, the pre-cursorsolutions, the basic solution, and/or the solution formed during andafter mixing the pre-cursor solutions and the basic solution may be heldat a temperature between about 40° C. and 150° C. In one or moreembodiments, the slurry resulting from the precipitation may be held ata temperature between about 40° C. and 150° C. for 1-24 hours after theprecipitation. In one or more embodiments, the basic solution may bemade from at least one of alkali metal carbonate, alkali metalbicarbonate, and alkali metal hydroxide dissolved in an aqueoussolution. In one or more embodiments, pre-cursors of a support materialmay also be dissolved in the initial pre-cursor solution andco-precipitated along with the pre-cursors of the plasmonic material andpre-cursors of the reactive component. In one or more embodiments, thepre-cursors of the plasmonic material, the reactive component, and thesupport material may be transition metal salts and they may be dissolvedin an aqueous solution.

In one or more embodiments, the metal salts are dissolved in thepre-cursor solutions to match the molar ratio of the desiredprecipitated compound. For example, when targeting the formation of aMg—Al hydrotalcite (Mg₆Al₂CO₃(OH)₁₆(H₂O)₄ support the precursor solutionis formulated to have a 3:1 molar ratio of Mg:Al. Additionally, thereare other hydrotalcites with a combination of a bivalent metal cationand a trivalent metal cation that may be used as support materials.Further, the ratio of the plasmonic material pre-cursor to the reactivecomponent pre-cursor may be tuned to match the molar ratio of metals inthe targeted surface alloy. In one or more embodiments, the molar ratioof metal in the plasmonic material to the metal in the reactivecomponent may be between 1000:1 to 10:1 or between 400:1 to 20:1.Finally, the amount of pre-cursors used may be tuned so that the supportmaterial may be between 99.9% to 20% by weight of the precipitate, orbetween 95% and 40% by weight of the precipitate, or between 90% and 60%by weight of the precipitate.

The co-precipitated particles may then be collected from solution (e.g.,by centrifugation, gravity sedimentation, etc.) and annealed at anelevated temperature to form the surface alloy particles. When theco-precipitated particles include a support material precipitatedtherewith, the annealing results in supported surface alloy particles.In one or more embodiments, the collected co-precipitated particles maybe washed by successive cycles of dispersion in water followed bycollection by centrifugation prior to the annealing. In one or moreembodiments, the annealing may be performed at least partially in areducing atmosphere. In one or more embodiments, the annealing isperformed initially in an inert atmosphere, followed by annealing in areducing atmosphere. In one or more embodiments, regardless of theatmosphere used the annealing may be performed at a temperature between200° C. and 1000° C. or between 400° C. and 700° C. In general, thehigher the temperature during the annealing process the larger themulticomponent plasmonic photocatalyst particles that will form.

In one or more embodiments, the reducing atmosphere may include acomponent that induces the segregation and enrichment of the reactivecomponent on the surface of the annealed particle to form a surfacealloy. Such a component may be referred to as a enrichment agent. Forexample, CO may be included in a reducing gas stream because CO maypreferentially bind to the reactive component over the plasmonicmaterial and the preferential binding can induce the segregation andenrichment of the reactive component at the surface to form a surfacealloy particle during the annealing. In one or more embodiments, theinclusion of H₂, NH₃, and hydrocarbons in the gas stream duringannealing may also function to segregate/enrich the reactive componenton the surface of the anneal particle to form a surface alloy. In one ormore embodiments, the annealing process may occur in the reactionchamber prior to catalytic reaction. That is, the surface alloymulticomponent photocatalyst may be formed by an activation step thatincludes annealing in the reaction chamber prior to catalytic reaction.

Lithographic and other deposition processes may also be employed to formmulticomponent photocatalysts. For example, colloidal lithography may beused to deposit a plasmonic material and a reactive component onto aninert substrate. By varying the deposition parameters, e.g., depositionangle and the resist thickness, the spacing between the plasmonicmaterial and the reactive component, and therefore the reactivity, ofthe multicomponent photocatalysts may be manipulated. Further,lithographic processes may be used to create arrays of multicomponentphotocatalysts. The lithographic and deposition processes that may beused to form multicomponent photocatalysts include, but are not limitedto, electron beam lithography, photolithography, atomic layerdeposition, chemical vapor deposition, thermal evaporation, nanoimprintlithography, templated growth, and sputtering.

The use of the multicomponent photocatalysts described herein is notintended to be particularly limited and, in general, the multicomponentphotocatalysts may be integrated into existing photocatalyst system andutilized similarly to any known photocatalyst. For example,multicomponent photocatalysts as described herein may be used in apack-bed reactor system, dispersed in a solvent, dispersed in a gasphase, or illuminated on a surface. In one or more embodiments, themulticomponent photocatalyst or a multicomponent photocatalystpre-cursor is processed into a pellet or film/thin layer prior toloading into the high-temperature reaction chamber. Such processing maybe accomplished by known methods. In some embodiments, themulticomponent photocatalysts may be dispersed onto a support materialprior to being loaded into a reaction chamber and being exposed tomolecular reactants for the particularly targeted reaction. The supportmaterial may generally include insulating and semiconducting materialsthat have minimal optical absorption in the visible spectrum. In one ormore embodiments, the support material may include aluminum oxides,silicon oxides, magnesium oxides, titanium oxides, zinc oxide, zirconiumoxide, tungsten oxide, iron oxides, calcium oxide and the like. In oneor more embodiments, the support material may be one selected fromcarbides, nitrides, sulfides, carbon materials, and two-dimensionaltransition metal dichalcogenides. In one or more embodiments, the carbonmaterial may be selected from activated carbon, charcoal, graphite,graphene, and graphene oxide. In one or more embodiments, the supportmaterial may be an aerogel. In one or more embodiments, themulticomponent photocatalysts may be dispersed on a support material ata weight percent between about 0.1 and 30 or at a weight percent betweenabout 0.1 and 5 percent. In other embodiments, the multicomponentphotocatalysts may be used as a catalyst even when they are notsupported on a support material.

In general, the reaction chamber will be engineered to allow forillumination of the multicomponent photocatalysts with a light source inorder to utilize the plasmon induced reactivity provided by themulticomponent photocatalysts. In one or more embodiments, theillumination may be tuned to be a wavelength on-resonance with the LSPR,leading to increased light absorption. Wavelength tuning may also beemployed to use specific wavelengths that may resonate with certainreactant or intermediate molecules, which may help control reactionselectivity. The net energy transfer from plasmon resonance tointermediates on the surface can induce a nonthermal desorption ofmolecules and intermediates that control the selectivity in chemicalreactions.

In one or more embodiments, the illumination may use high lightintensities, i.e., light intensities that are greater than the averageillumination of the sun (>100 mW/cm²), to provide more photon energy forthe system. In one or more embodiments, the reaction medium (i.e., theenvironment surrounding the reactants and the catalyst) is only heatedby photothermal heating induced by illuminating the contents of thereaction chamber. That is, in some embodiments there is no externalheating provided to the reaction medium in order to thermally drive thereaction. However, in one or more embodiments, the reaction medium maybe heated by an external heat source. For example, a heated gas may beflowed through the reaction chamber in order to externally heat thereaction medium. In embodiments where the reaction medium is a liquid,then the liquid may be externally heated. In general, heating candecrease reaction barriers at the catalyst surface, allowing lowerenergy photons to be utilized in any reaction taking place. In one ormore embodiments, the overall temperature within the reaction chambermay maintain a temperature below about 250° C., or below about 200° C.,or even below about 180° C. during the reaction. It is to be understoodthat, while the overall temperature may meet the above conditions, thecatalyst itself may be locally heated higher than above due tophotothermal heating. Moreover, in one or more embodiments, the reactionmedium may be externally cooled in order to control the temperature fromrising beyond a certain point. Further, in some embodiments thetemperature within the reaction chamber may be as high as 1000° C. andthe illumination may serve to further increase the efficiency of thethermocatalysis.

EXAMPLES

Synthesis of Aluminum Nanocrystals (Al NCs)

Al NCs with an average diameter of 100 nm with a 2 nm-4 nm self-limitingoxide surface layer were chemically synthesized. Briefly, 5 mL ofanhydrous tetrahydrofuran (THF) and 15 mL of anhydrous 1,4-dioxane(Sigma-Aldrich) were mixed in a 100 mL dry Schlenk flask under an Aratmosphere at 40° C. Under stirring, 6.5 mL of N,N-dimethylethylaminealane (0.5 M in toluene, Sigma-Aldrich) was injected into the reactionvessel, followed by rapid injection of 0.5 mL of 2 wt % Ti(O^(i)Pr)₄ intoluene. The color of the solution turned to brown within a few seconds,and to black/gray within an hour, indicating formation of Al NCs. Thereaction was allowed to proceed for two hours at 40° C., before beingremoved from the heat source. 1 mL of oleic acid was injected into themixture to quench the reaction. The as synthesized nanoparticles wereisolated by sonication and centrifugation at 2000 relative centrifugalforce (r.c.f.) in dry toluene, followed by three cycles of washing andcentrifuging in 2-propanol (IPA). Finally, Al NCs were dispersed in IPAand the solution purged by Ar and stored at room temperature for futureuse.

Example 1—Synthesis and Testing of Pd Decorated Al NCs

17.7 mg of PdCl₂ (anhydrous, 99.999% Sigma-Aldrich) were dissolved in 10mL of anhydrous acetonitrile (Sigma-Aldrich) over 24 hours to produce a0.01M solution of PdCl₂(MeCN)₂. 10 mL of 2-propanol and 5 mL ofpre-dispersed AlNC was placed into a 50 mL single-neck round bottomflask attached with a reflux condenser. The solution was brought toreflux before injecting 3 mL of PdCl₂(MeCN)₂. Reactions were refluxedbetween 10 and 60 minutes to yield Pd-decorated AlNCs. Nanoparticleswere isolated by centrifuging at 1000 r.c.f. and washing three timeswith acetonitrile before finally dispersing in 2-propanol.Alternatively, similar volumes of reagents can also be prepared at roomtemperature to decrease Pd coverage.

The upper two panels of FIG. 1 show a simplified model of a Pd—Al NC(reactive component-plasmonic component) multicomponent photocatalyst(left) and the calculated near-field enhancement in the Pd island forthe model multicomponent photocatalyst (right). The bottom two panels ofFIG. 1 show comparable models for a similarly sized Pd—Al₂O₃NC system.It is shown that the near-field enhancement in the Pd-island is over anorder of magnitude greater in the case of the Al NC model when comparedto the Al₂O₃ model, due to the plasmon induced near fields at the Al NCsurface.

FIG. 2 shows a high resolution transmission electron microscope (HRTEM)image showing a chemically synthesized Pd—Al NC. The Al NC with a thinAl₂O₃ shell separating the Pd deposited thereon is clearly depicted.FIG. 3 shows the optical properties of the chemically synthesized Pd—AlNC with increasing Pd coverage as predicted by theory (right) and asobtained experimentally (left). From top to bottom, pristine Al NCs witha dipolar plasmon resonance at 465 nm, 10 minutes refluxing with PdCl₂,30 minutes refluxing with PdCl₂, 60 minutes refluxing with PdCl₂.Increasing Pd coverage shows a redshift of the dipolar LSPR due to thereal part of the Pd permittivity and an increased damping of thequadrupolar mode at 300 nm due to the imaginary part of the Pdpermittivity. Further, TEM images showing a particle of each specimenthat is optically interrogated are shown in the same order in the middleof FIG. 3 (Scale Bar=50 nm).

Hydrogen-Deuterium Exchange Method—A customized stainless steelgas-phase high temperature reaction chamber (Harrick Scientific Product,Inc.) was used to mimic packed-bed reactor conditions using Pd-decoratedAl NCs prepared in refluxing IPA and loaded at 0.5 wt % on γ-Al₂O₃ as asupport. A combination of research purity H₂ and D₂ (Matheson TRIGAS;99.9999%) gases were flowed into the reaction chamber at each 15standard cubic centimeters per minute (sccm). HD production wasmonitored using a quadrupole mass spectrometer (Hiden Analytical Inc.)to continuously monitor HD (m/z=3) production in real time. Forwavelength and power dependent measurements, a tunable Ti:Sapphire laser(Coherent, Chameleon Ultra II, 150 fs, 80 MHz, 680-1080 nm, bandwidth ofapproximately 10 nm) equipped with a second harmonic generator(Angewandte Physik and Elektronik GmbH, output wavelength 350-530 nm)was used as a monochromatic light source. Wavelength-dependentmeasurements were performed at wavelengths with a minimum of 50 mW.Power-dependent measurements were performed at 492 and 800 nmcorresponding to the dipolar plasmon resonance and Al interbandtransitions, respectively, as measured using UV-Visible spectroscopy.Thermal activity was quantified in a similar manner, but through heatingwith a Harrick ATC-024-3 temperature controller.

FIG. 4 shows the photocatalytic activity for the hydrogen-deuteriumexchange reaction. More specifically, FIG. 4 shows the wavelengthdependence of H₂ desorption on Pd—Al NC (solid line connecting datapoints) and pristine Al NC (dashed line connecting data points) and thecalculated absorption cross-section of Pd—Al NC. The sample wasirradiated with a power density of 5 W/cm² at all wavelengths. As seenin FIG. 4, for the Pd—Al NC complex (solid line connecting data points),the wavelength dependence of HD production closely follows thecalculated absorption cross section supporting a hot-carrier mechanism.When qualitatively compared with pristine Al NCs (dashed line connectingdata point), the wavelength dependence of HD production is dramaticallydifferent, with the maximum HD production for the pristine Al NCsoccurring at a photoexcitation wavelength of 800 nm, corresponding tothe interband transition of Al.

FIG. 5 shows the quantitative consumption of H₂ at the dipolar LSPR ofPd—Al NC. Pd—Al NC multicomponent photocatalysts show nearly two ordersof magnitude greater reactivity than pristine Al NCs. More specifically,FIG. 5 shows the baseline levels of H₂ as monitored by massspectrometry. When 110 mW of 492 nm (dipolar resonance) light isilluminated onto a 0.5 wt % Pd—Al NC sample, the H₂ signal decreasesfrom 3.76×10⁶ c/s to 2.97×10⁶ c/s, meaning that 0.79×10⁶ c/s of H₂ wasconsumed in the hydrogen-deuterium exchange reaction. Accordingly,(0.79/3.76)×100%=21.0% of the H₂ is consumed. The total flow of H₂ intothe system was 15 sccm (standard cubic centimeters per minute),therefore the total consumption of H₂ is (15 mL/min×0.2101)/60 s=0.053mL/s. Under experimental conditions (25° C.; 1 atm) the molar volume ofH₂ would be 22.4 L/mol, so the total number of moles of H₂ transformedinto HD is 2.34×10−6 mol/s, which means 1.41×10¹⁸ H₂ have reacted.

FIG. 6 shows the excitation laser power dependence of the reaction usingthe Pd—Al NC multicomponent photocatalysts measured at 492 nm and 800nm, corresponding to the dipolar plasmon resonance and Al interbandtransition, respectively. The plot in FIG. 6 shows a supralinearresponse at both wavelengths. Such supralinear responses with increasingoptical power density have been suggested as a hallmark ofhot-carrier-driven chemistry on nanoparticle surfaces.

FIG. 7 shows temperature dependent reaction activity measurementsbetween 300 K and 400 K using the Pd—Al NC without externalillumination. These measurements show an increase in HD generation withincreasing temperature; however, the calculated wavelength-dependentlocal maximum temperature increase expected on the nanoparticle surfaceis only between 2 K and 16 K for Al and Pd surfaces, respectively,within the experimental range of excitation laser power densities. Suchsmall local temperature increases under illumination suggest that,although photothermal heating of the Pd lattice may contribute slightlyto H₂ desorption, the primary cause can be attributed to the excitationof photoexcited hot carriers in the multicomponent photocatalyst.

Acetylene Reduction—A customized stainless steel gas-phase hightemperature reaction chamber (Harrick Scientific Product, Inc.) was usedto mimic packed-bed reactor conditions using Pd decorated Al NCsprepared at room temperature and loaded at 0.5 wt % on γ-Al₂O₃ as asupport. N₂ (Matheson TRIGAS; 99.9999%), H₂ (Matheson TRIGAS; 99.9999%),and C₂H₂ (Praxair 5.02% in He), were flowed through the reaction chamberat 10.5, 0.5, and 4 sccm, respectively. The reduction of acetylene wasmonitored using a Shimadzu GC-2014 gas chromatograph connected directlyto the exhaust gas from the reaction chamber. Thermal activity wasquantified in a similar manner, but through heating with a HarrickATC-024-3 temperature controller.

The photocatalytic properties of the Pd—Al NC multicomponentphotocatalysts are translatable to other chemical reactions, such ashydrogenation. One important and industrially relevant reaction is theselective reduction of acetylene. Ethylene is a commodity chemicalprecursor used in the production of polyethylene-based materials withwidespread commercial use; however, under traditional thermalconditions, ethane is also produced in a side reaction duringhydrogenation of acetylene. As shown in FIG. 8, with Pd—Al NCmulticomponent photocatalysts, we have found a drastic increase in theselective reduction of acetylene to ethylene under white-lightillumination (data points at higher selectivities at 4.8 W/cm² andbeyond) when compared to traditional thermal reduction (data points withlower error bars whose selectivity drops off at ˜360 K). The selectivityalso shows a large increase with increased laser power density. Anincrease in ethylene:ethane product ratio from ˜7 to ˜37 is observed forthe photo-hydrogenation case. In contrast, traditional thermal heatingof the Pd—Al NC complexes showed that ethylene:ethane selectivityleveled off at a maximum of ˜10:1 before showing a drop to ˜6:1 at 360 K(black).

FIG. 9 shows representative gas chromatogram yields for bothphoto-hydrogenations (A) and thermal hydrogenations (B) using the Pd—AlNC multicomponent photocatalysts. Photo-hydrogenation yields (A) show anincrease with laser power, but an overall limited yield likely due tostarving the surface of dissociated H₂ at high powers. Thermalhydrogenation yields (B) show notably higher yields at 360 K, the sametemperature associated with a decrease in ethylene selectivity (See,FIG. 8).

That selectivity enhancement is seen in photo-hydrogenation, yet notseen in traditional thermal hydrogenation, is likely due to theavailability of dissociated H₂. In both photo-hydrogenation and thermalhydrogenation cases, acetylene adsorbs on the surface and undergoes thefirst and second hydrogenations to produce ethylene. At this point, twoforward reaction pathways are possible: ethylene desorption orsubsequent hydrogenation of ethylene to produce ethane. Both desorptionand hydrogenation of ethylene from Pd(111) are known to have similaractivation barriers within the margin of error of previous DFTcalculations. Therefore, the availability of dissociated H₂ dictates thebranching ratio between these two reaction pathways.

In photocatalytic hydrogenations, plasmon induced hot carriers lead torapid desorption of H₂, biasing the equilibrium toward desorption andthus limiting the availability of hydrogen on the surface for additionalhydrogenation of ethylene. The hypothesis of hot-carrier-inducedH-starved surfaces leading to increased selectivity is also backed up byreduced yields of ethylene in the photocatalytic hydrogenation case(See, FIG. 9 plot (A)). With illumination, there is lesssurface-activated H₂ which also reduces the likelihood of the first andsecond hydrogenations of acetylene needed to produce ethylene. Inthermal hydrogenations, ethylene yields are higher (T>360 K) at theexpense of reduced selectivity, most likely due to minimal changes indissociated H₂ surface coverage, and enough kinetic energy in the systemto overcome the activation energies and favor subsequent hydrogenationsof ethylene. The selectivity increase observed for thephoto-hydrogenation of acetylene could open doors for developing moreselective hot-carrier-driven chemistry.

Example 2—Synthesis and Testing of Al@Cu₂O

For the synthesis of Al@Cu₂O, 2.5 mL of as synthesized Al NCs (1 mg/mLin IPA) were transferred to an oven-dried Schlenk flask and the totalvolume of the solution adjusted to 10 mL using IPA. The reactionsolution was degassed at room temperature for about an hour and thenunder Ar atmosphere the flask was heated to reflux. While refluxing, 1mL of 0.01M fresh Cu (II) acetate (99.999% trace metal-basis,Sigma-Aldrich) in dry acetonitrile was rapidly injected into thereaction with constant stirring. The reflux continued for 2 hours toyield Al@Cu₂O nanoparticles. The as-synthesized nanoparticles wereisolated by centrifuge at 2000 r.c.f. and washed three times with IPA,and finally dispersed in IPA.

FIG. 10 shows TEM images of the Al@Cu₂O particles formed by the chemicalsynthesis. In (a) a TEM image of as synthesized Al NCs, while (b) showsa TEM image after the growth of the Cu₂O shell around the Al core. Inboth (a) and (b) the scale bar is 50 nm. In (c) a HRTEM image is showndepicting the Al@Cu₂O showing more clearly the Al core, the thinamorphous Al₂O₃ oxide layer, and the surrounding Cu₂O shell.

FIG. 11 shows results for the optical characterization of Al NCs,Al@Cu₂O, and Cu₂O. More specifically, FIG. 11 shows experimental (left)and theoretical (right) UV-Vis extinction spectra of Al NCs, Al@Cu₂O,and Cu₂O in IPA. Pristine Al NCs show a dipolar LSPR around 460 nm thatredshifts to around 550 nm after growth of the Cu₂O shell (typicalthickness of the is about 15-20 nm) due to real part of the Cu₂Opermittivity.

The photocatalysts used in this study were prepared from a homogeneousdispersion of plasmonic particles dispersed on a high surface areaγ-Al₂O₃ support at 5 wt %. Photocatalytic measurements were performedusing about 20 mg of this sample mixture, loaded into a customizedstainless steel chamber with a quartz window to allow for illumination(Harrick Scientific Product Inc.) that mimics continuous flow packed-bedreactor conditions. High purity H₂ and CO₂ at a total pressure of 1 atmand a total flow of 10 standard cubic centimeters per minute (sccm) wereflowed continuously into the chamber. The chamber outlet was connectedto a gas chromatograph (Shimadzu Inc.). A supercontinuum fiber laser(Fianium, 450-850 nm, 4 ps, 40 MHz) and a tunable Ti:sapphire laser(Coherent, Chameleon Ultra II, 150 fs, 80 MHz, bandwidth ˜10 nm) wereused as light sources.

FIG. 12 shows gas chromatogram results (a) of photocatalytic CO₂hydrogenation experiments, via the reverse water-gas shift reaction(rWGS), performed in CO₂ and H₂ at a 1:1 ratio and a total flow rate of10 sccm and also separately in He (10 sccm) atmospheres under visiblelight illumination. The spectrum of the light source used forillumination is shown in (b). CO formation was detected when CO₂ and H₂were both present, upon illumination of the Al@Cu₂O multicomponentphotocatalyst/γ-Al₂O₃ mixture. Illumination of the photocatalyst in aninert He atmosphere did not produce any measurable product, verifying COformation was not from the degradation of the oleic acid capping agent.Also, there was no measurable product in pure γ-Al₂O₃ in the absence ofthe multicomponent photocatalyst, verifying that the Al@Cu₂O plasmonicphotocatalyst was the active component.

FIG. 13 shows plots of the photocatalytic versus thermal-driven activitycharacterization for the rWGS reaction using the Al@Cu₂O. In (a) gaschromatogram of the reaction products output from the reaction chamberduring light-induced (illuminated with 7 W/cm²) and purely thermallydriven (heated to 350° C.) rWGS is shown. Prior to the reactions usingillumination, the multicomponent photocatalyst and stainless steel stageare in thermal equilibrium with room temperature. Upon, illuminationwith 7 W/cm² visible light, the temperature of pure oxide supportreaches up to 55° C., while after loading plasmonic nanoparticles intothe oxide support, the temperature rises up to slightly above 150° C.under the same light intensity.

The results shown in (a) show that in contrast to the highly selectiveCO formation observed for the photocatalytic process, the thermallydriven rWGS reaction (when the photocatalyst is used without externalillumination) results in the formation of both CH₄ and CO. In (b) theselectivity for CO formation over CH₄ formation as a function oftemperature is shown for Al@Cu₂O during thermally driven rWGS (i.e., noillumination). At 200° C., very low selectivity of about 40 and 55% wereobtained on Al@Cu₂O. As the temperature increased, the selectivity of COover CH₄ increases, as formation of CH₄ is an exothermic reaction. Theselectivity of CO/CH₄ formation reaches up to 97% at 400° C. However,even at this high of a temperature the CO formation selectivity in thethermal process is still less than that of 100% selectivity obtained(see, the results in FIG. 13(a)) from plasmon-induced process at loweroperating conditions. In (c) the overall rate of product formation onAl/γ-Al₂O₃ and Al@Cu₂O/γ-Al₂O₃ as a function applied temperature in thepurely thermal process (no illumination). Also in (c), the reaction rateduring the light-induced process (illumination using 10 W/cm²) onAl/γ-Al₂O₃ and Al@Cu₂O/γ-Al₂O₃ are shown as the two data pointsencircled in the ellipsoid at the corresponding recorded temperature ofabout 160-170° C.

The results in (c) show that a photothermal effect does not play a majorrole in CO formation because, as shown, the onset temperature of productformation in a purely thermal process is around 200° C. Indeed, theoverall reaction yield at an illumination intensity of 10 W/cm² iscomparable to the thermal process at temperatures of 400° C. Thus, theresults in (c) provides additional evidence that plasmon-inducedchemical transformations can operate more efficiently and selectivelyunder milder reaction conditions.

FIG. 14 shows (a) the rate of CO formation as a function ofvisible-light intensity under ambient conditions and (b) the externalquantum efficiency (EQE) as a function of photon flux. The rate of COformation catalyzed by Al@Cu₂O is significantly higher than that ofpristine Al without the reactive Cu₂O shell, particularly at higherillumination intensities. Similarly, the Al@Cu₂O heterostructuresexhibit higher EQE. The positive dependence of EQE to incident photonflux observed for both systems is a distinct feature of plasmon-inducedcharge-carrier driven photocatalysis. On the contrary, it is known thatincreasing irradiation intensities does not improve EQE on semiconductorsurfaces. In conventional semiconductor photocatalysis, reaction rate isproportional to intensity^(n), with n<1, whereas for plasmon-inducedphotocatalysis by hot carriers n>1. ‘N’ is a descriptor that iscalculated from experimental power dependence measurements and itdescribes the relationship between photon input and the reaction rate.Higher ‘n’ means that there is a higher efficiency on a photon-by-photonbasis. For example, in (b), n was calculated to be ˜2.65 and ˜3.78 on Aland Al@Cu2O, respectively.

FIG. 15 shows a plot of the measured EQE for Al@Cu₂O and pristine Alversus illumination wavelength. It was found that coating Al with a Cu₂Oshell substantially enhanced the EQE. This catalytic enhancement isparticularly pronounced around the dipolar plasmon resonance of Al@Cu₂Oat ˜570 nm, thus supporting a plasmon-enhanced carrier generationmechanism for driving rWGS.

Example 3—Synthesis and Testing of Cu—Ru Surface Alloy@Cu SupportedCatalyst

Cu—Ru Surface Alloy@Cu Supported on MgO—Al₂O₃ (19.5 at % Cu & 0.5 at %Ru):

0.707 g (2.925 mmol) Cu(NO₃)₂.3H₂O (Sigma-Aldrich®, #61194), 0.0190 g(˜0.075 mmol) RuCl₃.xH₂O (Acros organics, # A0324917), 2.308 g (9 mmol)Mg(NO₃)₂.6H₂O (Sigma-Aldrich®, #63084) and 1.125 g (3 mmol)Al(NO₃)₂.9H₂O (Sigma-Aldrich®, #237973) were dissolved in 15 mL DI water(Milli-Q® Advantage A10) to make the metal precursor solution. A second,basic solution was prepared by dissolving 2.544 g (24 mmol) anhydrousNa₂CO₃ (J.T.Baker®, #3602-01) in 20 mL DI water.

10 mL of DI water was added to a 100 mL 5-neck, round-bottom flask andheated to 80° C. The metal precursor solution and Na₂CO₃ solution wereadded simultaneously and in a dropwise fashion to the preheated water.The pH was monitored with a pH meter (Accumet® Portable, AP63) and keptat ˜pH=8 by varying the speed of addition both solutions, which wascarried out over 15 minutes. The resulting solid slurry was allowed tostir at 80° C. for 24 hours before cooling to room temperature. Thecatalyst precursor was isolated by centrifuging the slurry at ˜100 g andsubsequently washed 4 times with DI water and dried in the air at 120°C. overnight.

To activate the catalyst prior to any measurements, the dry precursorwas packed into the high-temperature reaction chamber (HarrickScientific Products Inc., # HVC-VUV-5, quartz window) within a 2mm-inner diameter stainless steel sample ring to get a thick,cylindrical sample pellet. After purging the chamber with 200 sccm(standard cubic centimeter per min, at 70° F. and 1 Bar) He for 10 minsto expel excess air, the precursor was annealed at 500° C. with a ramprate of 10° C./min and held for 1 h in 20 sccm He (Airgas, ultrahighpurity, 99.999%). Then, the gas was switched to 10 sccm H₂ (Airgas,research purity, 99.9999%) to reduce the sample at 500° C. for one hour.FIG. 19 shows a high-resolution transmission electron micrograph (TEM)of a single Cu—Ru surface alloy particle resulting from the reducingprocess described above. FIG. 20 shows a high-angle annular dark-field(HAADF) image of the reduced Cu—Ru surface alloy (lighter contrastareas) supported on the MgO—Al₂O₃ support. FIG. 21 shows a plot of thesize distribution of the reduced Cu—Ru surface alloy. Forthermocatalysis experiments, precursor was packed into the chamberwithout using a sample ring to get a thin sample pellet so that thetemperature of the whole sample was uniform.

Cu Nanoparticles Supported on MgO—Al₂O₃ (20 at % Cu):

The preparation and treatment procedure was the same as above for theCu—Ru surface alloy, but the metal precursor solution was prepared bydissolving 0.725 g (3 mmol) Cu(NO₃)₂.3H₂O, 2.308 g (9 mmol)Mg(NO₃)2.6H₂O and 1.125 g (3 mmol) Al(NO₃)₂.9H₂O in 15 mL DI water. FIG.22 shows a HAADF image of the reduced Cu nanoparticles (lighter contrastareas) supported on the MgO—Al₂O₃ support. FIG. 23 shows a plot of thesize distribution of the reduced Cu nanoparticles.

Ru nanoparticles supported on MgO—Al₂O₃ (0.5 at % Ru): 0.0190 g (0.075mmol) RuCl₃.xH₂O, 2.870 g (11.19 mmol) Mg(NO₃)₂.6H₂O and 1.399 g (3.73mmol) Al(NO₃)₂.9H₂O were dissolved in 15 mL DI water to make the metalion mixed solution. The preparation and treatment procedure was the sameas for Cu—Ru surface alloy sample. FIG. 24 shows a HAADF image of thereduced Ru nanoparticles (lighter contrast areas) supported on theMgO—Al₂O₃ support. FIG. 25 shows a plot of the size distribution of thereduced Ru nanoparticles.

FIG. 33 is a table showing the element concentration in thecoprecipitated precursor (i.e., before the activation process) asmeasured by inductively coupled plasma (ICP) spectroscopy.

FIG. 26 shows UV-Vis diffuse reflectance spectra of Cu—Ru surface alloy(solid line), Cu nanoparticles (dashed line) and Ru nanoparticles(short-dashed line). Vertical axis is the Kubelka-Munk function.

FIG. 27 shows powder X-ray diffraction (PXRD) of Cu—Ru surface alloy onMgO—Al₂O₃ support and XRD data of Cu, Ru, MgO and Al₂O₃ fromInternational Centre for Diffraction Data (ICDD) cards. The diffractionpattern shows five peaks corresponding to metallic copper and threepeaks/shoulder (labeled with *) matching with (111), (200), (220) ofMgO. No peak corresponding to crystalline Al₂O₃ is found, indicating itsamorphous structure. There is no peak corresponding to Ru either becauseof the low loading of Ru.

FIG. 28 shows X-ray photoelectron spectroscopy (XPS) result of Cu—Rusurface alloy supported on MgO—Al₂O₃ and Ru nanoparticles supported onMgO—Al₂O₃. Higher surface atomic percentage of ruthenium in Cu—Rusurface alloy supports the surface alloy structure by showing a surfaceenriched in Ru.

FIG. 29 shows a plot of highest surface temperatures and average surfacetemperatures of a sample pellet of the Cu—Ru surface alloy supported onMgO—Al₂O₃ under white light illumination as a function of lightintensity. The reactor temperature was kept at room temperature for thedata presented in FIG. 29.

Catalysis Experiments—Ammonia Decomposition (2NH₃→N₂+3H₂)

Photocatalysis reactions were carried out in fixed-bed, continuum-flowreactor (Harrick Scientific Products, Inc., # HVC-VUV-5). White lightfrom a supercontinuum laser (Fianium, WL-SC-400-8, 400-900 nm, 4 ps, 80MHz) was focused by an achromatic lens with a 100 mm focal length(Thorlab, AC254-100-A-ML) resulting in an ˜2 mm diameter beam profile onthe catalyst surface. The temperature of the chamber was maintained at27° C. unless otherwise noted. The feed gas was pure NH₃ (Airgas,anhydrous purity, 99.99%). Gas flow rates were controlled with mass flowcontrollers (Alicat Scientific). The flow rates were optimized fordifferent experiments based on two criteria: (i) high enough to make theconversion below 2% to achieve differential reactor conditions accordingto a flow-rate-dependence experiment; (ii) as low as possible whilemaintaining high signal to noise ratios. All the catalytic reactionswere operated under atmospheric pressure. The effluent composition wasmonitored by an online quadruple mass spectrometer (MS) (HidenAnalytical Inc., QIC-20) at m/e=2 (H₂), 28 (N₂) and 17 (NH₃) in realtime or an online gas chromatography (GC) (Shidmazu-2014) equipped witha pulsed discharge helium ionization detector (PDHID) and a molecularsieve 13X (MS-13X) packed column. MS can detect both of reactant (NH₃)and products (N₂&H₂) while GC can only detect products with the columnwe used. But GC gives better signal to noise ratios.

Reaction rates were quantified based on linear calibration curves ofpure H₂ and N₂ for both MS and GC. As the conversion is controlled below2%, the increase of total volume flow due to reaction stoichiometry(2NH₃ converted to 3H₂ and 1N₂) is negligible. The reaction rate wascalculated according to the following equation:

r _(abs) (μmol·s−1)=Δp (%)·f (sccm)60(s·min⁻¹)·22400 (ml·mol⁻¹)·106(μmol·mol⁻¹)

where Δp is the percentage change of a reactant or product in the flowwhile f is the flow rate of feeding NH₃

The specific reaction rate is calculated based on the mass of precursor:

r ₀ (μmol·g⁻¹·s⁻¹)=r _(abs) (μmol·s⁻¹)m _(precursor) (g)

The turnover frequency (TOF) is calculated based on the followingformula:

TOF_(R) (h⁻¹)=r _(abs) (μmol·s⁻¹)/n _(Ru) (μmol)·3600 (s·h⁻¹)

where n_(Ru) is the moles of ruthenium in the catalyst, which isobtained from ICP-MS measurement.

FIG. 16 shows a plot of the H₂ production rate during photocatalysis(9.6 W/cm² white light illumination) and thermocatalysis (at 482° C.)for the various catalysts. The left column for each catalyst representsthe photocatalysis results, while the thermocatalysis results are shownin the right column. Compared to monometallic Cu and Ru nanoparticles,the photocatalytic reaction rate on the Cu—Ru surface alloy@Cu supportedon MgO—Al₂O₃ catalyst was ˜20 and ˜177 times higher, respectively. Forillumination at 9.6 W/cm², without external heating, the photocatalyticreaction rate of Cu—Ru surface alloy@Cu supported on MgO—Al₂O₃ can reachas high as 1.2 mmol H₂/g/s. The reaction rate dropped to zero when lightwas turned off. The turnover frequency (TOF) based on Ru loading was >15s⁻¹ and the energy efficiency (the reaction is endothermic with ΔH^(o)_(r×n) of 46 kJ/mol) was calculated to be ˜18% from:

$\eta_{energy} = {\frac{{reaction}\mspace{14mu} {rate}\mspace{14mu} \left( {{mol}\text{/}s} \right)*{reaction}\mspace{14mu} {enthalpy}\mspace{14mu} \left( {J\text{/}{mol}} \right)}{{optical}\mspace{14mu} {power}\mspace{14mu} \left( {J\text{/}s} \right)}*100\%}$

FIG. 17 show a plot demonstrating the reaction rate during multiple hourlong measurement of photocatalytic rates using the Cu—Ru surfacealloy@Cu supported on MgO—Al₂O₃ catalyst under 9.6 W/cm² white lightillumination without external heating. The ratio of photocatalyticreaction rates based on the measured amounts of NH₃, N₂ and H₂ areconsistent with the stoichiometry of the reaction, confirming theabsence of unintended side reactions.

FIG. 18 shows a comparison of photocatalytic and thermocatalytic ratesusing the Cu—Ru surface alloy@Cu supported on MgO—Al₂O₃ catalyst. Thehorizontal axis corresponds to the surface temperature of the catalystdue to photothermal heating in the upper set of measurements(photocatalysis) or external heating in the lower set of measurements(thermocatalysis). The light intensity differences between successivedata points are 0.8 W/cm². FIG. 18 shows that when ammonia decompositionwas performed without illumination, but with external heating attemperatures equivalent to those achieved under illumination, thethermocatalytic rates of H₂ production were 1-2 orders of magnitudebelow the observed photocatalytic rates.

FIG. 30 shows two Arrhenius plots of apparent activation barriers fordifferent wavelengths (upper plot) under constant intensity of 3.2 W/cm²and in the dark (trend line not marked by a wavelength) and (lower plot)for different light intensities at 550 nm and in the dark (trend linenot marked by a wavelength).

FIG. 31 shows the wavelength dependence of photocatalytic reaction rateon Cu—Ru surface alloy@Cu supported on MgO—Al₂O₃. Feeding rate of NH₃was 5 sccm and 1.5 mg photocatalyst was used.

FIG. 32 shows the apparent activation barriers (eV) under variousillumination conditions for the Cu—Ru surface alloy@Cu supported onMgO—Al₂O₃.

In the plasmonic photocatalytic decomposition of ammonia, the apparentactivation barrier depends strongly on both incident wavelength andlight intensity. This dependence can be accounted for by hotcarrier-induced associative desorption of N₂, which simultaneouslyreduces the coverage of reaction intermediates, significantly decreasingthe apparent activation barrier. A knowledge of the light-dependentactivation barrier can be used to quantitatively predict photocatalyticreaction rates for given reaction conditions, such as illumination andexternal heating. The predictive and quantitative methodology presentedhere paves the way for optimization of plasmonic photocatalysis forenergy efficient applications.

Catalysis Experiments—Methane Dry Reforming (CH₄+CO₂→2CO+2H₂)

Cu—Ru surface alloy@Cu supported on MgO—Al₂O₃ and Cu supported onMgO—Al₂O₃ catalysts were prepared by a coprecipitation method asdescribed above. The photocatalysts were denoted as Cu_(x)Ru_(y), with xand y referring to the respective atomic percentage of Cu and Ruelements in total metal elements (Cu, Ru, Mg and Al) of the catalyst.All surface alloy photocatalyst samples fabricated exhibit similar sizedistributions, with an average diameter of the Cu—Ru surface alloyparticles of ˜5 nm. From XPS it can be observed that the binding energyof Ru 3p shifts to a higher value compared to metallic Ru, an indicationof electron transfer from Ru to Cu. Also, the surface atomic ratio ofRu/Cu is higher than the bulk atomic ratio determined from ICP-MS,suggesting the enrichment of Ru on the surface. For Cu₂₀ nanoparticlessynthesized without Ru, the concentration of surface Cu was 94.8 μmol/g,measured by N₂O chemisorption, while for surface alloy nanoparticles theconcentration of surface Cu decreased with increasing additions of Ru inthe synthesis, supporting the realization of formation of a Ru—Cusurface alloy. In addition, samples with Ru loading below 0.2 at % havea Ru surface coverage <20% according to N₂O chemisorption, lower thanthe maximum allowable surface coverage for atomic dispersion on aclose-packed surface. UV-Vis diffuse reflectance spectra show a resonantpeak at ˜560 nm (similar to that shown in FIG. 26), attributed to thedipolar localized surface plasmon resonance of Cu nanoparticles for allsamples, with only a slight broadening due to the damping effect of Ruadatoms on the Cu nanoparticle plasmon.

FIG. 34 shows the reaction rate and long-term stability duringphotocatalytic dry methane reforming reaction under 19 W/cm² white lightillumination. The arrow in FIG. 34 indicates the relative ordering forthe listed catalysts by CH₄ reaction rate. FIG. 35 shows the selectivityand long-term stability during photocatalytic dry methane reformingreaction under 19 W/cm² white light illumination. The arrows in FIG. 35point the label to the respective plot. The plots for theCu_(19.8)Ru_(0.2) and the Cu_(19.9)Ru_(0.1) samples overlap at theuppermost selectivity. During the experiments the reactor temperaturewas kept at room temperature and selectivity is defined as the ratio offormation rate of H₂ to CO.

For pure copper nanoparticles (Cu₂₀), an initial reaction rate of ˜50μmol CH₄/g/s under 19 W/cm² white light illumination was detected. Butthe activity quickly decayed to only ˜4 μmol/g/s after 5 h reaction.Coke deposition on the surface of nanoparticles strongly correlated withphotocatalyst deactivation. A black substance formed on the surface ofthe photocatalyst pellet soon after light excitation, which wasidentified as amorphous carbon by Raman spectroscopy. Though the surfacetemperature at the hottest spot of catalyst pellet due to light-inducedheating was measured to be ˜750° C. under current experimentalconditions, Ostwald ripening of the nanoparticles is not significant,since the size distribution of the nanoparticles after photocatalysiswere measured to barely change.

Notably, an extremely low fraction of Ru (Cu_(19.95)Ru_(0.05)) wasobserved to increase the initial photocatalytic reaction rate by ˜2.5times (128 μmol/g/s), greatly improving the stability, with ˜90%activity maintained after a continuous 5 hour experiment. Furthermore,an unprecedented stability was achieved for both Cu_(19.9)Ru_(0.1) andCu_(19.8)Ru_(0.2) catalysts, with 100% efficiency maintained over a20-hour photocatalytic reaction. Even after 50 hours, no decay wasobserved at all for the Cu_(19.9)Ru_(0.1) sample. There is no appearanceof carbonaceous species in the Raman spectra of the spent catalysts andthe increase of carbon content was negligible from Element Analysis. Rusites are more reactive for methane dissociation than the pure coppersurface, as predicted by DFT calculations, where the atomic dispersionof reactive sites suppresses C—C bond formation and concomitant cokingby isolating the surface carbon intermediates. For theCu_(19.95)Ru_(0.05) sample, the surface coverage of Ru was too low and asubstantial part of the reaction was catalyzed by the exposed coppersurface, which is vulnerable to coking. Further increase of Ru loading(Cu_(19.5)Ru_(0.5)) gave a higher initial photocatalytic reaction rateas expected, but the stability was compromised, with 13% ofphotocatalytic activity lost after a 16 hour reaction. This is likelydue to increased Ru concentration, because the Ru atoms start to formsurface islands at this coverage, where the carbon intermediates canpolymerize to form coke.

FIG. 36 shows the long-term stability (solid circles) and selectivity(open circles) of photocatalysis under 19 W/cm² white light illuminationand thermocatalysis at 1000 K reactor temperature when using theCu_(19.8)Ru_(0.2) catalyst. The initial reaction rate of thermocatalysisof Cu_(19.8)Ru_(0.2) at 727° C. in the dark was only ˜60 μmol CH₄/g/s,which is less than 25% of the photocatalytic reaction rate with 19 W/cm²white light illumination and no applied heating (˜275 μmol CH₄/g/s).Considering the comparable surface temperature at the hottest spot(˜750° C.) and the gradient distribution of temperature in the volumeheated by light illumination, we propose that hot-carrier-mediatedchemical reaction is the major mechanism in this photocatalysis process.The thermocatalytic reaction rate observed was less stable than thephotocatalytic reaction rate, decaying to ˜4 μmol CH₄/g/s after 8 h.From TEM, Raman spectroscopy and Elemental Analysis, it was concludedthat coke deposition, rather than sintering, is likely the cause of theinstability in the thermocatalysis, despite the single-atom-alloysurface, revealing that hot carriers are important for coke resistance.The low selectivity of the thermocatalyzed reaction indicates thatsurface-adsorbed H is inclined to react with CO₂ to decrease the surfaceabundance of oxygen intermediates, which suppresses the removal ofadsorbed carbon from the surface through oxidative gasification(C(a)+O(a)→CO(g)). In other words, the kinetic mismatch between theformation and gasification rates of surface carbon results inaccumulated coke deposition.

On the other hand, in photocatalysis, hot carriers may enhance theassociative desorption of H₂, as reflected by the high observedselectivity, and consequently maintain the surface abundance of oxygenintermediates for surface carbon removal. Additionally, hot carrierscould probably enhance the direct reaction between CO₂ and absorbed C onRu sites through the reverse Boudouard reaction (C+CO₂→2CO), which has arelatively high reaction barrier in the ground state and is hardlyfeasible through phonon excitation.

FIG. 37 shows a plot of the intensity dependence of the photocatalyticreaction rate and selectivity using the Cu_(19.8)Ru_(0.2) catalyst.Error bars represent the standard deviation of measurements of threedifferent batches of sample. FIG. 38 shows a plot of the wavelengthdependence of the photocatalytic reaction rate and selectivity using theCu_(19.8)Ru_(0.2) catalyst. Light intensity of 3.5 W/cm² was used forall wavelengths. Error bars represent the standard deviation ofmeasurements by the gas chromatograph at the same spot of the same batchof sample. For all the photocatalytic experiments shown in FIGS. 34-38,1.5 mg catalyst was used and 2 mm light spot was shined onto the surfaceof sample pellet. The reactor temperature was kept at room temperature.For the thermocatalytic experiments in FIG. 36, 5-10 mg catalyst wasused and the experiments were performed in the dark.

The apparent activation barrier (E_(app)) for methane dry reforming onthe Cu_(19.8)Ru_(0.2) photocatalyst was measured to be 0.85 eV fromArrhenius fitting of thermocatalytic reaction rates at differenttemperatures. The selectivity shows a V-shaped dependence ontemperature, with a transition temperature at ˜800 K (527° C.).Initially, the selectivity decreases from a maximum value of about 0.3H₂/CO with increasing temperature in the temperature region of 650-800K, probably because the reaction rate of the RWGS side reaction rises upfaster with temperature compared to that of the methane dry reformingreaction. However, the Gibbs free energy of the RWGS side reactionbecomes less negative and constrains its reaction rate at highertemperature (T>800 K). The theoretical lower limit of the selectivitypredicted by thermodynamics reproduces the experimental values in thethermodynamics-controlled region (T>800 K) quite well. On the contrary,the selectivity of photocatalysis (FIG. 37) increases monotonically withlight intensity, reaching ˜100% for intensity above ˜10 W/cm². Thedramatic contrast of the absolute values and the trend in selectivitybetween thermocatalysis and photocatalysis confirms the dominant role ofa hot-carrier-mediated mechanism in the photocatalysis process.

The wavelength dependence plots shown in FIG. 38 shows that the highestreaction rate and selectivity were observed when the sample wasilluminated by light of 550 nm wavelength, the plasmonic resonantwavelength of the nanoparticles. For wavelengths longer than 550 nm,both of the reaction rate and the selectivity decreased because fewerhot carriers can be generated. For shorter wavelength (λ<550 nm), thoughthe absorption remained high, the reaction rate and selectivity alsodecreased. The portion of absorption attributed to the interbandtransition of Cu is substantial in this wavelength region, increasing asthe wavelength decreases while the hot carriers produced from interbandtransitions are less energetic compared to the surface plasmon-derivedhot carriers. The reactivity increases again at 450 nm, which weattribute to the synergistic effect of photothermal heating and surfaceplasmon-derived hot carriers. Although the number of effective hotcarriers from plasmon decay decreases in the wavelength region of 475nm-450 nm, the surface temperature is increased due to higher overallabsorption: this could significantly enhance hot-carrier activation andsubsequent chemical reactivity at these wavelengths. However, thiseffect is less substantial for associative desorption of H₂, since theselectivity decreases monotonically with shorter wavelength, probablybecause temperature has only a minor effect on the kinetics of anelementary step with a small activation barrier. The wavelengthdependence suggests that hot carriers derived from surface plasmon decayare mainly responsible for the hot-carrier-mediated mechanism and theeffect of temperature is more apparent when surface plasmon-derived hotcarriers are rare, for example at the interband transition region.

The direct coupling of plasmonic materials with reactive particles intoa single multicomponent plasmonic complex allows for absorptionenhancements in and/or hot carrier transfer to poorly light-absorbingreactive components. With multicomponent plasmonic photocatalysts asdescribed herein, hot-carrier production and photothermal heating can bedramatically increased near catalytically active surfaces. This conceptis a highly modular one; for example, tuning the composition or size ofthe plasmonic material allows for light-induced photocatalysis atspecific wavelengths of the electromagnetic spectrum, enablingoptimization of such multicomponent plasmonic complexes for specificchemical reactions and reaction pathways. Likewise, by changing thereactive component to different metals, alloys, semiconductors, orinsulators, the surface chemistry and photocatalytic activity can behighly tuned. Multicomponent plasmonic photocatalsysts may increasehot-carrier production, thereby allowing for new, light-driven reactionpathways on a reactive component attached thereto. Developing themulticomponent plasmonic concept to favor specific hot carrier-drivenphotocatalytic processes where control over reaction specificities ishighly desirable and opens a new door for the development of precise,ultimately predictive control of catalytic chemistry using light.

Advantageously, the multicomponent plasmonic photocatalysts of thepresent disclosure alleviates the issue that plasmonic materials haverelatively inert surfaces for most chemical reactions and for substratebinding by combining the plasmonic materials with another catalyticparticle or particles or atoms having more reactive surfaces or sitesresulting in increased reactivity and selectivity compared totraditional single component catalysts and sensors. The direct couplingof plasmonic materials with reactive particles into a singlemulticomponent plasmonic complex allows for absorption enhancements inpoorly light-absorbing reactive components. With multicomponentplasmonic photocatalysts as described herein, hot-carrier production andphotothermal heating can be dramatically increased near catalyticallyactive surfaces. This concept is a highly modular one; for example,tuning the composition or size of the plasmonic material allows forlight-induced photocatalysis at specific wavelengths of theelectromagnetic spectrum, enabling optimization of such multicomponentplasmonic complexes for specific chemical reactions and reactionpathways. Likewise, by changing the reactive component to differentmetals, alloys, semiconductors, or insulators, the surface chemistry andphotocatalytic activity can be highly tuned. Multicomponent plasmonicphotocatalsysts may increase hot-carrier production, thereby allowingfor new, light-driven reaction pathways on a reactive component attachedthereto. Developing the multicomponent plasmonic concept to favorspecific hotcarrier-driven photocatalytic processes where control overreaction specificities is highly desirable and opens a new door for thedevelopment of precise, ultimately predictive control of catalyticchemistry using light. A transition from extreme, high-temperatureconditions to low-temperature activation of catalytically activetransition metal nanoparticles could have widespread impact,substantially reducing the current energy demands of heterogeneouscatalysis.

Although only a few example embodiments have been described in detailabove, those skilled in the art will readily appreciate that manymodifications are possible in the example embodiments without materiallydeparting from this invention. Accordingly, all such modifications areintended to be included within the scope of this disclosure as definedin the following claims.

What is claimed:
 1. A method of making a multicomponent photocatalyst,comprising: inducing precipitation from a pre-cursor solution comprisinga pre-cursor of a plasmonic material and a pre-cursor of a reactivecomponent to form co-precipitated particles; collecting theco-precipitated particles; and annealing the co-precipitated particlesto form the multicomponent photocatalyst comprising a reactive componentoptically, thermally, or electronically coupled to a plasmonic material.2. The method of claim 1, further comprising dissolving a pre-cursor ofa plasmonic material and a pre-cursor of a reactive component into asolution to form the pre-cursor solution.
 3. The method of claim 2,wherein a pre-cursor of a support material is also dissolved into thesolution.
 4. The method of claim 1, wherein precipitation is induced bycontacting the pre-cursor solution with a basic solution.
 5. The methodof claim 4, wherein the basic solution comprises at least one of alkalimetal carbonate, alkali metal bicarbonate, and alkali metal hydroxidedissolved in an aqueous solution.
 6. The method of claim 1, wherein thepre-cursor of the plasmonic material and the pre-cursor of the reactivecomponent are transition metal salts.
 7. The method of claim 1, whereinthe molar ratio of metal in the pre-cursor of the plasmonic material tometal in the pre-cursor of the reactive component is between 1000:1 to10:1.
 8. The method of claim 3, wherein the co-precipitated particlesare between 99.9% and 20% support material.
 9. The method of claim 1,wherein the annealing is performed at least partially in a reducingatmosphere.
 10. The method of claim 1, wherein the annealing isperformed at a temperature between 200° C. and 1000° C.
 11. A method ofcatalyzing a reaction, comprising: forming a multicomponentphotocatalyst pre-cursor, by a method comprising: inducing precipitationfrom a pre-cursor solution comprising a pre-cursor of a supportmaterial, a pre-cursor of a plasmonic material, and a pre-cursor of areactive component to form co-precipitated particles of themulticomponent photocatalyst pre-cursor; and collecting themulticomponent photocatalyst pre-cursor; loading the multicomponentphotocatalyst pre-cursor into a high-temperature reaction chamber;annealing the loaded multicomponent photocatalyst to form amulticomponent photocatalyst comprising a reactive component optically,thermally, or electronically coupled to a plasmonic material; feedingreactants into the reaction chamber; and illuminating the multicomponentphotocatalyst in the reaction chamber with a light source having awavelength overlapping a plasmon resonance of the plasmonic material.12. The method of claim 11, wherein the multicomponent photocatalystpre-cursor is processed into a pellet or film prior to loading into thehigh-temperature reaction chamber.
 13. The method of claim 11, whereinthe pre-cursor of the plasmonic material and the pre-cursor of thereactive component are transition metal salts.
 14. The method of claim11, wherein the molar ratio of metal in the pre-cursor of the plasmonicmaterial to metal in the pre-cursor of the reactive component is between1000:1 to 10:1.
 15. The method of claim 11, wherein the annealing isperformed at least partially in a reducing atmosphere.
 16. The method ofclaim 11, wherein the annealing is performed at a temperature between200° C. and 1000° C.
 17. The method of claim 11, wherein the plasmonicmaterial is selected from gold (Au), silver (Ag), copper (Cu), aluminum(Al), and alloys including said elements.
 18. The method of claim 11,wherein the reactive component is selected from palladium (Pd), platinum(Pt), ruthenium (Ru), rhodium (Rh), nickel (Ni), iron (Fe), cobalt (Co),iridium (Ir), osmium (Os), titanium (Ti), vanadium (V), indium (In). 19.The method of claim 11, wherein the multicomponent photocatalyst has thereactive component alloyed at the surface of the plasmonic material. 20.The method of claim 11, wherein the reaction is one of methane steamreforming, methane dry reforming, ammonia decomposition, nitrous oxidedecomposition, reverse water gas shift, water gas shift, reduction ofacetylene, ammonia synthesis, and Fisher-Tropsch synthesis.